Methicillin-resistant staphylococci are a leading cause of nosocomial infections worldwide and are commonly resistant to multiple antimicrobial agents. The accessory genetic element, SCCmec, harbors the methicillin resistance determinant and is thought to be horizontally transferred between the principal staphylococcal pathogen, Staphylococcus aureus, and commensal coagulase-negative species, such as S. epidermidis. Recombinations of ubiquitous core genes are also thought to occur within these species, but the frequency, relative size, and biological consequence of recombination in staphylococci is poorly understood. Our long-term goals are to understand how genetic variation contributes to the ecological and evolutionary success of bacterial pathogens. In this application, we investigate the extent to which horizontal genetic transfer has influenced genetic variation in staphylococci. Through sequence-based typing of a prospective collection of clinical isolates of staphylococci, we will test the hypotheses that S. epidermidis recombines it core genes more frequently than S. aureus, and that rare, large intraspecies recombinations result in hybrid S. aureus clones (Aim 1). Through an understanding of genetic background and a molecular characterization of SCCmec, we will test the hypothesis that our local methicillin-resistant S. aureus arise through acquisition of SCCmec from global isolates of the same species, not local congenerics (Aim 2). Moreover, through application of an innovative PCR scanning tool to diverse isolates, we will test the hypothesis that poorly characterized S. aureus of clinical importance harbor novel accessory genes (also Aim 2). Finally, we will examine a potential biological consequence of horizontal genetic transfer in staphylococci. Through a molecular characterization of the ica locus, we will test the hypothesis that genetic background and SCCmec are both independent predictors of biofilm phenotype (Aim 3).